Tens of thousands of migrants have been stuck on their journey north across Europe

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A man sets up a tent at the premises of the old Athens international airport, which is used as a camp for about 2,800 people, in southern Athens, on August 4, 2016. The migrant and refugee arrivals at the Greek islands of north Aegean sea, rise slowly but steadily after the failed coup in Turkey. More than 57,000 refugees and migrants remain stranded in Greece.

With Turkey in crisis and Europe's borders closed, smugglers in northern Greece are expecting a profitable summer.

Greek police say traffickers are using increasingly sophisticated methods — motorcycle spotters, maps of border surveillance "blind spots," and even police informants — to move out refugees who have been stuck in this Greek border town for months.

Higher smuggling fees and steadily worsening odds of success have done little to dent the determination of migrants like Sorah Rahimi.

The 22-year-old psychology student traveled from Afghanistan with his mother who is in poor health — and is an ideal target for smuggling rings that are re-emerging in the region. He agreed to pay traffickers 2,500 euros ($2,750) to travel from Greece's northern border with Macedonia to Sweden, but only made it a few miles before being spotted by police and turned back.

"We need to get to Sweden. Our whole family is there. We no longer have anything in Afghanistan," Rahimi told the AP before being taken to a migrant shelter in northern Greece.

Tens of thousands of migrants have been stuck on their journey north across Europe. Governments across the continent slammed their borders shut in March to avoid a repeat of the mass migration in 2015, when more than a million refugees and migrants arrived, triggering a string of political upheavals.

The trip north has become increasingly risky and time-consuming. Rahimi spent three months at the port of Piraeus, near Athens, before traveling north by train.

Last month's attempted coup in Turkey, followed by a draconian government crackdown there on perceived government opponents, has added to the uncertainty.

Traffickers in Greece have re-appeared in greater numbers since the government cleared a massive makeshift camp at Idomeni on the Greek-Macedonian border in late May, according to officials from the country's border guard, national police and security division interviewed by the AP.

In a recent operation, police arrested 29 alleged members of two trafficking rings in northern Greece. One gang had received information from a Greek police officer on gaps in the night vision camera network on the Greek-Turkish border.

Police said the smugglers brought some 600 migrants into Greece from Turkey over several months and helped them reach other countries, using a fleet of taxis, scout vehicles, prepaid "burner" cellphones, and a system of code words that included: "dogs" for police, "garbage trucks" for police cars, and "cement blocks," ''fish," or "kebabs" when referring to the migrants themselves.

"At least six (Greek) taxi drivers were involved in the smuggling rings, charging a regular fare while knowingly participating in the illegal activity," the Greek Police's Brig. George Pantelakos said. "Every transfer assisted by scouts on motorcycle, in an effort to avoid detection."

Other suspects were from Syria, Iraq, Armenia, Albania and Kazakhstan.

Five police officers have also been arrested in Macedonia, accused of helping smugglers north of the Greek border.

In Greece alone, more than 57,000 migrants are stranded, most staying at about 60 government-run camps around the country.

The European Union's law enforcement agency Europol analyzed data from anti-smuggling operations over the first six months of 2016. It found fees charged last year by traffickers for the entire trip from Syria and other warzones to the EU are now often set as the rate for crossing a single border.

Migrants are also more likely to be held up for months and become vulnerable to exploitation.

"Last year, the trips were sometimes completed in one to two weeks; now a journey can last for months," the agency said. "While in 2015, 0.2 percent of migrants declared that they had to work to pay back smugglers, this rose to 5 percent in 2016."

When the numbers peaked in late March, 14,000 people were camped out at Idomeni. Now all that remains there are aluminum posts used for emergency lighting. Sunflowers have appeared in fields that a few weeks ago were packed with tents.

But the migration problem has not been resolved. Dozens of migrants are still hiding in cornfields, under bridges and in forests near Idomeni. Since the refugee camp was cleared, between 50 and 100 migrants a day are spotted trying to cross the border and being turned back, according to police estimates.

Some are found hidden in vans, buses, cars or trains. Others try their luck on foot, looking for gaps in the razor-wire border fence along river banks and mountain trails.

Lina Siopi, a 44-year-old from Idomeni, owns a coffee shop outside the village railway station and says migrants are routinely found hidden in freight cars and detained.

"We see it all the time. In the latest incident ... a family was hiding under a sheet of tarp (in a freight carriage). I don't know where they were from: a couple with two sons," she said.

In Thessaloniki, Greece's second largest city, one hour's drive south of the Macedonian border, smugglers brazenly negotiate terms with migrants camped out at parks near the city's main train station. They have the upper hand, since most migrants have already been in the country for several months.

Afghan refugee Fatima Davoodi has agreed on a price already and is waiting for smugglers to pick up her, her husband and two sons, aged six and 10.